Aluminum oxide LPCVD system

ABSTRACT

A process and apparatus for Al 2  O 3  CVD on silicon wafers using aluminum tri-isopropoxide in a high-volume production environment is presented. The conditions required to use ATI in a production environment and provide maximum utilization of ATI are first of all delivery of ATI via direct evaporation. The ATI source bottle is pumped out (bypassing substrates) until propene and isopropanol signals are reduced to 1% of process pressure before start of aluminum oxide deposition. Either IR spectroscopy or mass spectrometry can be used to provide a control signal to the microprocessor controller. Heating the supplied tetramer to 120° C. for two hours assures complete conversion to trimer. The ATI is stored at 90° C. to minimize decomposition during idle periods and allow recovery of trimer upon return to 120° C. for two hours. During periods of demand, the ATI is held at 120° C. to minimize decomposition.

FIELD OF THE INVENTION

The present invention generally relates to the chemical vapor deposition(CVD) of dense thin aluminum oxide (Al₂ O₃) films on silicon substratesin the manufacture of semiconductor devices and, more particularly, toan improved method and apparatus for the low pressure chemical vapordeposition (LPCVD) of aluminum oxide films using aluminumtri-isopropoxide Al(OC₃ H₇), known in the art as ATI, as a precursor toaluminum oxide in a high volume manufacturing environment.

BACKGROUND OF THE INVENTION

The utility of CVD Al₂ O₃ as a reactive ion etch (RIE) stop layer(during tungsten contact stud patterning) has been clearly demonstrated.For this application, a dense aluminum oxide film is defined as havingthe following characteristics:

Index of refraction: 1.59-1.62, at 632.8 wavelength

Etch rate (3.6% phosphoric acid at 75° C.): <25 nm/min

Hydroxyl content (OH, by infrared (IR) spectroscopy, absorbance permicron of film thickness at 3500 cm⁻¹): <0.005

Shrinkage of film thickness upon annealing (30 min. at 500° C.): <1.5%

The literature indicates that aluminum tri-isopropoxide (Al(OC₃ H₇)),known in the art as ATI, is a viable sub-500° C. Al₂ O₃ precursor. See,for example, J. A. Aboaf, J. Electrochem. Soc., 114, 948 (1967), J.Fournier et at., Mat. Res. Bull., 23, 31 (1988), J. Kwon, J. Saraie andY. Yodogawa, J. Electrochem Soc., 132, 890 (1985), H. Mutoh et al., J.Electrochem. Soc., 122, 987 (1975), and R. W. J. Morssinkhof et al.,"Mechanistic Aspects of the Deposition of Thin Alumina Films Depositedby MOCVD", paper presented at Spring Meeting of the Materials ResearchSociety, San Francisco, Calif. (1990). In these references, ATI isutilized under conditions in which 1) only bubbling is used to deliverATI to the reactor, 2) a variety of ATI operating temperatures (78°-170°C.) is practiced, 3) deposition repeatability is not demonstrated, and4) only a single substrate requires coating. However, while theliterature describes the use of ATI for Al₂ O₃ deposition, theliterature describes bubbling techniques for delivery of ATI but failsto present a workable manufacturing process. In fact, the bubblingtechnique described in the literature will not work in a manufacturingenvironment for the reasons discussed in more detail below.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processof chemical vapor deposition (CVD) of dense Al₂ O₃ on silicon wafersusing aluminum tri-isopropoxide in a high-volume production environment.

It is another object of the invention to provide a process and apparatusfor aluminum oxide CVD which insure growth rate repeatability.

It is a further object of the invention to provide an apparatus whichmaximizes the utilization of ATI source material in a LPCVD aluminumoxide process.

According to the invention, there is provided a method to use aluminumtri-isopropoxide (ATI) for chemical vapor deposition of aluminum oxidein a high volume manufacturing environment in which the ATI is storedduring periods of no production demand at a temperature no less than 90°C. but not greater than 95° C. and the ATI temperature is raised to anominal temperature of 120° C. in preparation for a period of productiondemand. During periods of production, the temperature of the ATI ismaintained at a temperature not greater than 125° C. A critical featurein the practice of the method according to the invention is that the ATIis transported to the reaction chamber by direct evaporation or liquidinjection without bubbling. The ATI is directed to a reactor bypass inlieu of to the CVD reactor during a start up period, the flow throughthis bypass is maintained until the concentration of ATI is determinedto be sufficiently pure, typically greater than 99%. When theconcentration of the ATI in the reactor supply line has reached theprescribed level, the ATI is directed to the CVD reactor. A chamberconditioning step may then be induced to minimize "first wafer" filmdegradation due to competition between the chamber and wafer for thereactant ATI. The temperature of the ATI source is reduced to about 90°C. upon termination of the production demand period.

According to another aspect of the invention, an apparatus is providedfor depositing aluminum oxide on a substrate, which apparatus performsthe foregoing method. The apparatus includes a pressure vesselcontaining aluminum tri-isopropoxide source material from which a sourcechemical vapor is produced. A delivery manifold is connected to thepressure vessel and selectively delivers the source chemical vapor tofirst or second outputs. An isothermal oven containing the pressurevessel and the delivery manifold heats the same. Pressure andtemperature within said pressure vessel are monitored. A chemical vapordeposition chamber is provided with gas injector nozzles for injectionof gas into the chamber. An insulated and temperature controlled exhaustline is connected to the first output of said delivery manifold and tothe chemical vapor deposition chamber to provide a temperaturecontrolled exhaust from the delivery manifold and the chemical vapordeposition chamber. A temperature controlled delivery line is connectedat a first end thereof to the second output of the delivery manifold. Atemperature controlled pressure differential type flow controller (suchas an MKS Model 1151B) is connected to the second end of the deliveryline for providing accurate flow of said source chemical vapor throughthe delivery line. A premix chamber, including a temperature controlledvalved premix manifold assembly, is connected to the pressuredifferential mass flow controller. The premix manifold assembly hasfirst and second outputs, the first output of the premix manifoldassembly comprising a chamber for allowing the source chemical vapor tobe mixed with a mass flow controlled, preheated gas. The first output ofthe premix manifold assembly is connected to the gas injectors of thechemical vapor deposition chamber for injection of the mixed sourcechemical vapor and gas therein. The second output of the premix manifoldassembly is connected to the exhaust line. A controller is connected tothe delivery manifold, heating device, the process vessel monitoringsensors, the exhaust line, the delivery line, the pressure differentialmass flow controller, and the premix manifold assembly for controllingthe pressure and the temperature of source chemical vapor beingdelivered throughout the same to be substantially constant therebyminimizing potential condensation and/or decomposition of the sourcechemical vapor during delivery. The controller additionally controls thedelivery of the source chemical vapor from the pressure vessel to theexhaust line, the delivery line, the premix manifold assembly, and thechemical vapor deposition chamber according to prescribed depositionprocess parameters.

According to another aspect of the invention, parameter settings areselected for operation of the chemical vapor deposition chamber that isprovided for depositing aluminum oxide on a substrate. The parametersettings are chosen such that an aluminum oxide film of high density isdeposited repeatably and efficiently for a low temperature fabricationapplication (less than 450° C.). An efficient deposition is one whereindeposition occurs on the workpiece only and not on the ATI deliverysystem plumbing or the reactor chamber walls. A repeatable depositionprocess is one where the thickness and quality of the film do not varyfrom batch to batch.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a graph showing short term ATI decomposition as effluentpressure versus temperature;

FIG. 2A is a graph of the infrared (IR) spectra of the initial ATIeffluent from the bottle, and FIG. 2B is a graph of the IR spectra ofthe flowing ATI effluent from the bottle after the decompositionproducts have been removed;

FIGS. 3A, 3B and 3C are graphs of the nuclear magnetic resonance (NMR)spectra of the ATI tetramer to trimer conversion under differingconditions;

FIG. 4 is a block diagram showing the major components of the apparatusfor depositing thin films of aluminum oxide on wafers according to apreferred embodiment of the invention; and

FIG. 5 is a flow diagram showing the control of the apparatus shown inFIG. 4 by the microprocessor controller.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION AND

BEST MODE FOR PRACTICE OF THE INVENTION

Generation of ATI vapor by means of bubbling is not a feasible method todeliver a controlled amount of this precursor gas to the processreactor. While bubbler systems are typically operated under conditionswhere the carrier gas is said to be "saturated" with the vapor of thecondensible precursor, the amount of vapor which is actually entrainedin a carrier stream depends on a number of factors. These parametersinclude the temperature of the storage vessel as well as its thermalstability, the level of liquid in this vessel, and the flow rate andtemperature of the carrier gas. A change in any of these parameters willaffect the concentration of precursor which is delivered to the processreactor. Physical and chemical properties which are specific to ATI makedelivery of the vapor of this material by means of bubbling even moredifficult. The aluminum isopropoxide molecule is a solid at roomtemperature and must be heated to 120° C. to achieve a nominal 1 Torrpressure to allow its delivery to the reaction chamber. At this constanttemperature, ATI is moderately unstable and decomposes into volatileproducts whose vapor pressures are greater than that of the parentmaterial, see analytical data shown in table below and in FIG. 1.Partial decomposition also results in changes in the liquid phaseleading to a reduction of the vapor pressure of the ATI.

                  TABLE                                                           ______________________________________                                        EFFECT OF T ON VAPOR PRESSURE                                                 AND DECOMPOSITION                                                                          Vapor pressure                                                                            % decomposed (30                                     Storage T (°C.)                                                                     (torr)      days)                                                ______________________________________                                        120          1.0          .60                                                 130          2.0         1.1                                                  140          4.5         2.0                                                  ______________________________________                                    

In addition, ATI has the tendency to serf-associate, and it formsdimeric, trimeric and tetrameric phases, each of which has a differentvapor pressure. The presence of decomposition products, the inherentlylow vapor pressure of ATI and changes in the phase of the material makebubbling impractical for the delivery of ATI vapor to the processreactor.

Control of the delivery of aluminum tri-isopropoxide (Al(OC₃ H₇)₃) thatis used in the CVD aluminum oxide process is difficult due to ATI's 1)trimer/tetramer phase window and 2) slow thermal decomposition.ATI-based CVD process development was conducted at InternationalBusiness Machines Corp. (IBM) using direct evaporation from an ATIsource bottle. Bubbling was judged inconsistent with the achievement ofhigh throughput batch process. A "single wafer-like" system withindividual substrate temperatures and flow control to six stations wasselected for process development.

During development, poor growth rate repeatability was observed. Overlong periods, the growth rate pattern manifested as a gradual rise ingrowth rate over a series of four to six runs (within a day) to amaximum and an eventual decrease of this daily maximum over a period ofseveral weeks as the ATI source bottle was depleted. In addition, agradual rise in ATI source bottle pressure was observed over periods asshort as an hour (depending on the source temperature).

The poor growth rate reproducibility suggests that ATI is notsufficiently stable under the wide variety of temperatures, as issuggested in the CVD literature. There is in fact a tendency of ATI todecompose at higher temperatures. IR and mass spectroscopy have beenused to identify the volatile ATI decomposition products as isopropanoland propene, as shown in FIG. 2. A possible decomposition sequence is

    2Al(OC.sub.3 H.sub.7).sub.3 →Al.sub.2 (OC.sub.3 H.sub.7).sub.2 (OH).sub.4 +Propene

    Al.sub.2 (OC.sub.3 H.sub.7).sub.2 (OH).sub.4 →Al.sub.2 (O).sub.2 (OH).sub.2 +2Isopropanol

The aluminum tri-isopropoxide hydroxide and oxide hydroxide areinvolatile.

In the literature, decomposition of ATI is reported to occur only attemperatures above 170°-200° C., and none of the reported CVD workindicates any difficulty in controllably delivering ATI. This stands indirect contrast to the instability observed at IBM when operating ATI attemperatures well below 170° C. Over the short term, stable growth ratescould only be achieved after pump out of ATI decomposition products evenwhen operating ATI as low as 115° C.

The fall off of decomposition rate with temperature suggests that aminimum storage (no ATI demand) and operating (ATI demand) temperaturebe used. However, a selection of these minima is complicated by ATI'strimer/tetramer equilibria and the need for sufficient vapor pressure toallow 1.0 sccm flow per wafer. At least 1 sccm of ATI is required toachieve a minimum allowable growth rate of 150 Å/min. Formation oftrimer/tetramer mixture with a high tetramer content (>5%) must beavoided as the tetramer is not volatile. Since ATI is supplied as asolid in tetramer form, determination of a minimum operating temperaturerequired to convert the tetramer to trimer was required.

For operation, a temperature of 120° C. was found sufficient toguarantee complete conversion to trimer in two hours and providesufficient vapor pressure to allow a nominal 1.0 sccm flow per wafer.However, a minimum storage temperature can not be selected at random dueto ATI's trimer and tetramer phase equilibria. Prolonged storage attemperatures below 80° C. causes changes in melting point that likelyindicate phase transformation to the involatile tetramer. However,prolonged storage (e.g., 30 days) at 900° C. does not prevent recoveryof the trimer (two hours at 120° C.). Results of an investigation of thetetramer to trimer conversion using NMR spectroscopy are shown in FIGS.3A, 3B and 3C. Therefore, by storing at 90° C., decomposition duringoften lengthy idle periods can be minimized without irreversiblyaltering the ATI phase.

As a result of slow thermal decomposition, propene and isopropanol areevident in the gas phase. During formation of these gas phase species,irreversible changes also occur in the ATI melt. Elemental analysis ofthe melt remaining, after using 85-90% of an initial charge, indicatethat the melt is aluminum rich (16.3% versus 13.2% expected for ATI).This suggests existence of a catenated system of Al--O--Al linkages.This Al--O--Al linkage-containing material is miscible with ATI due toits similar nature. During storage/operation, as the ATI content of themelt falls due to evaporation and slow decomposition, the available ATIvapor pressure will fall as predicted by Raoult's law. As a result, theflow delivered from a source container operating without constrictionwill drop with time. A CVD system operating under such a maximum-flowprotocol (either by direct evaporation or bubbling) will exhibit growthrates that are out of control. Such a process would not be operable in aproduction environment.

Referring now to FIG. 4 of the drawings, there is shown an apparatus fordepositing aluminum oxide on a substrate according to a preferredembodiment of the invention. A pressure vessel 11 contains aluminumtri-isopropoxide (ATI) source material from which a source chemicalvapor is produced. A delivery manifold 12, including a pair of valves12₁ and 12₂, is connected to the pressure vessel 11 and has first andsecond outputs respectively connected to these two valves. The valves12₁ and 12₂ of the delivery manifold selectively deliver the ATI sourcechemical vapor to the first or second output. An isothermal oven 13contains the pressure vessel 11 and the delivery manifold 12 for heatingthem and maintaining them at an accurate, controlled temperature.Sensors 14 and 15 respectively monitor pressure and temperature withinthe pressure vessel 11.

A chemical vapor deposition (CVD) process chamber 16 has walltemperature control by the process controller 17, as indicated bycontrol line 18. The CVD process chamber 16 also has at least one gasinjector nozzle 19 for injection of gas into the chamber and a heatedsusceptor or chuck 20 which holds a workpiece 10, such as a siliconwafer substrate, on which aluminum oxide films are to be deposited.Insulated and temperature controlled exhaust lines 21₁, 21₂, and 21₃ arerespectively connected to the first output of said delivery manifold 12and to CVD process chamber 16 for providing a temperature controlledexhaust from the delivery manifold and the CVD process chamber to avacuum pump 22. A temperature controlled delivery line 23 is connectedat the first end thereof to the second output of said delivery manifold12. The delivery line 23 has a cross section so as not to restrict themass flow of the ATI source chemical vapor for a desired deposition rateof aluminum oxide on a wafer. A temperature controlled pressuredifferential mass flow controller (MFC) 24 is connected to the secondend of the delivery line 23 and, under the control of process controller17, provides a controlled flow of the ATI source chemical vapor throughthe delivery line 23.

A premix chamber 25 including a heated and insulated valve premixmanifold assembly 26 is connected to the pressure differential mass flowcontroller 24. The premix manifold assembly 26 has first and secondoutputs. The first output of said premix manifold assembly 26 comprisesa chamber for allowing the ATI source chemical vapor to be mixed with amass flow controlled, preheated gas, such as argon from source 27. Thisgas is delivered via mass flow controller (MFC) 28 to the premixmanifold assembly 26, preferably at a rate of 200 sccm. The first outputof the premix manifold assembly 26 is connected by line 29 to the gasinjector nozzle 19 of the CVD process chamber 16 for injection of themixed ATI source chemical vapor and other gas therein. The second outputof the premix manifold assembly 26 is connected to the exhaust line 21₃.

The process controller 17 is a microprocessor based controller and isconnected to the delivery manifold 12, the oven 13, the pressure vesselmonitoring sensors 14 and 15, the exhaust lines 21₁, 21₂ and 21₃, thedelivery line 23, the mass flow controller 28 and pressure differentialmass flow controller 24, and the premix manifold assembly 26 forcontrolling the pressure and the temperature of the ATI source chemicalvapor being delivered throughout the system to be substantially constantthereby minimizing potential condensation and/or decomposition of theATI source chemical vapor during delivery. The process controller 17further controls the delivery of the ATI source chemical vapor from thepressure vessel 11 to the exhaust lines 21₁, 21₂ and 21₃ , the deliveryline 23a and 23b, the premix manifold assembly 26, and the CVD processchamber 16 according to prescribed deposition process parameters.

The delivery line 23 leads to an infrared (IR) cell 30. A Fouriertransform infrared (FTIR) spectrometer 31, or other IR sensing device,proximate to the IR cell 30 analyzes the composition of vapor flowingthrough the IR cell 30 before it travels to the chemical vapordeposition chamber 16. The FTIR spectrometer 31 sends information to theprocess controller 17 which controls the valve of the premix manifoldassembly 26. The process controller 17 operates the valve of the premixmanifold assembly 26 based upon the information it receives from theFTIR spectrometer 31. The delivery line 23 leads from the IR cell 30through the mass flow controller (MFC) 24 to the valve of the premixmanifold assembly 26. The outputs from this valve are a line 29 leadingto the CVD process chamber 16 and the exhaust line 21₃ leading to thesystem pump 22.

When the reactant vapor flowing in the IR cell 30 is analyzed by theFTIR spectrometer 31, information is given to the process controller 17indicating whether contaminants or decomposition products are present. Acomposition of reactant vapor which is harmful to the reactor orworkpiece within the reactor will cause the process controller 17 tooperate the valve of the premix manifold assembly 26 to switch such thatthe ATI chemical source vapor will proceed through exhaust line 21₃ topump 22, rather than through line 29 to the chemical vapor depositionchamber 16. If the composition of the ATI chemical source vapor isappropriate, the valve of the premix manifold assembly 26 will beoperated to direct the ATI chemical source vapor through line 29 tonozzle 19 in the CVD process chamber 16 and proceed to reaction onworkpiece 10.

The process controller 17 operates to control the temperature of the ATIin pressure vessel 11 during periods of no production demand at atemperature at no less than 90° C. but not greater than 95° C. Duringperiods of no production demand, the ATI is stored under vacuum. Theprocess controller 17 raises the ATI temperature to a temperature nogreater than 125° C. in preparation for a period of production demandand maintains the ATI during periods of production demand at atemperature not greater than 125° C. The ATI chemical vapor istransported to the CVD process chamber 16 via direct evaporation. Toimprove delivery of the ATI chemical vapor during periods of productiondemand, the ATI pressure vessel 11 may be agitated. The ATI chemicalvapor is directed to the reactor bypass exhaust line 21₃ in lieu ofnozzle 19 in CVD process chamber 16 during a start up period andmaintaining flow to the bypass exhaust line 21₃ until the concentrationof ATI chemical vapor in the delivery line 23 as measured by the IR cell30 and FTIR Spectrometer 31, is greater than a prescribed level,typically greater than 99%. The ATI chemical vapor is directed to nozzle19 in the CVD process chamber 16 through line 29 after the concentrationof ATI chemical vapor in the reactor supply line has reached aprescribed level, typically greater than 99%, for deposition of aluminumoxide on the workpiece 10. The temperature of the ATI chemical pressurevessel is again reduced to about 90° C. upon termination of theproduction demand period.

FIG. 5 is a flow diagram showing the process controlled by themicroprocessor based process controller 17. On start up, the processcontroller 17 determines whether there are wafers ready for processing,as indicated by decision block 41. If not, the system idles at functionblock 42 until wafers are present for processing. Once wafers are readyfor processing in CVD process chamber 16, the oven 13 temperature israised to 125° C. and stabilized, as indicated by function block 43.Then, the ATI is flowed through delivery line 23 to the IR cell 30 and,from there, via the valve in premix manifold assembly 26 to the exhaustline 21₃, bypassing the process chamber 16 function block 44. Based onthe output of FTIR spectrometer 31, the process controller 17 determinesin decision block 45 whether the concentration of the reactant gas issufficiently pure. If it is, the valve in the premix manifold assembly26 is operated to flow the ATI vapor to the CVD process chamber 16 infunction block 46. The flow of ATI to the CVD process chamber 16continues until processing of the wafers is completed, as determined indecision block 47. At that point, the valve in premix manifold assembly26 is actuated, the oven 13 temperature is reduced to 90° C., asindicated in function block 48, and pump out the ATI pressure vessel viaexhaust line 21₁ is accomplished during idle.

The literature references discuss a variety of reactor chamber walltemperatures from walls with no temperature control at all to thosecontrolled at 225° C. to those that are at the same temperature as theworkpiece (so called hot-walled systems). In addition, although some ofthe literature demonstrates deposition of high quality film, thesereferences accomplish this at the expense of efficiency of deposition(by allowing deposition on the reactor walls in a hot walled case wherehomogeneous gas phase reaction will occur leading to high particulateformation) or at the expense of reduce workpiece capacity (by depositingat very low growth rates). Workpiece capacity will also be reduced inthe hot wall case because deposition on the reactor leads to periodiccleaning of the reactor that reduces uptime. Furthermore, none of theliterature references teaches how to repeatably deposit highly densealuminum oxide via CVD using ATI. None of the literature referencesindicates the importance of maintaining both the reactor chamber walland delivery plumbing temperatures in a tightly controlled range. Noneof the references discusses the repeatability performance of thethickness and film quality of the systems for film deposition taught bythese references.

Operation in different pressure regimes is also discussed in theliterature. Operation at atmospheric pressure is undesirable due to atendency toward gas phase nucleation and resultant foreign materialcontamination to the workpiece. Although another reference demonstratesa process at 750 mTorr, even lower operating pressures are required forthe exercise of a high volume manufacturing process due to therequirement that the ATI source be maintained at a temperature below120° C.

Parameter settings of the CVD chamber that are critical in achieving theefficient and repeatable deposition of highly dense aluminum oxide rimsat low workpiece temperatures and at high throughput include tightlycontrolled temperatures of the chamber wall and reactant gas injectorsystem and a low chamber pressure. The temperature of the chamber wallsand gas injectors must not be so low as to allow condensation upon thesecomponents but must not be so high as to allow decomposition of the ATIduring delivery. Decomposition during delivery would not only reduce theefficiency of the use of the ATI, but would also alter the surfaces ofwetted components giving rise to foreign material contamination andfurther sites for surface reactions. The efficient and repeatabledelivery of ATI to the workpiece is based upon a minimization of ATIlosses during transport and control of the condition of the surfacesthrough which the ATI is transported before reaching the workpiece. Atlow workpiece temperatures great care must be provided to avoid deliveryof non-ATI species to the workpiece surface due to the known tendency ofaluminum oxide to trap impurities such as water. For higher workpiecetemperatures (above 500° C.), the tendency for impurities such as waterto be trapped by the growing aluminum oxide film will be reduced.However, there will be a greater tendency for other impurities such ascarbon to be trapped. The application of ATI for deposition of aluminumoxide at low workpiece temperatures precludes the use of warm-walled CVDchamber (where the chamber walls will be at temperatures between 135° C.and 225° C. or greater) because products of the decomposition reactionoccurring on the walls of the chamber may interfere with the depositionof high density films on the workpiece. Hot-walled systems suffer frominefficient usage of the ATI precursor and the need to periodicallyclean the reactor walls reducing the system's throughput or periodicreplacement of the reactor chamber raising the system cost.

Low pressure is required for the use of an ATI delivery system that isbased upon direct evaporation and to reduce the incidence of gas phasereaction that may also produce decomposition products that may interferewith the deposition of high density films on the workpiece or produceparticulate contamination.

The temperature of the chamber walls should be controlled at atemperature greater than the temperature of the ATI vessel but limitedto a temperature of 5° C. above the ATI vessel and preferably at atemperature no higher than 125° C. Consistent with the use of directevaporation to deliver the ATI, the pressure of the chamber should beless than the vapor pressure of the ATI at the ATI vessel temperatureand should preferably be less than 200 mTorr.

In summary, the conditions required to use ATI in a productionenvironment and provide maximum utilization of ATI according to thepresent invention are:

1. Delivery of ATI via direct evaporation. Since the vapor pressure ofthe ATI melt varies with time, the use of bubbling does not provide aconstant partial pressure of ATI. With direct evaporation, one canthrottle back on the requested flow so that the delivery will beinsensitive to vapor pressure changes.

2. Pump out of ATI source bottle (bypassing substrates) until propeneand isopropanol signals are reduced to less than some prescribed level,preferably less than 1% of process pressure before start of aluminumoxide deposition. Either IR spectroscopy or mass spectrometry can beused to provide a control signal to the microprocessor controller.

3. Heating as supplied tetramer to 120° C. for two hours to assurecomplete conversion to trimer.

4. Storage of ATI at 90° C. to minimize decomposition during idleperiods and allow recovery of trimer upon return to 120° C. for twohours.

5. Operation of ATI between 115° and 125° C. and preferably at 120° C.to minimize decomposition during periods of demand.

6. Controlling the temperature of the reactor chamber, the ATI deliveryplumbing, and the reactant gas injector system to a temperature lessthan 5° C. above the ATI vessel temperature and preferably less than125° C.

7. Operation of the reactor chamber at a pressure of less than the ATIvapor pressure at the ATI vessel temperature and preferably at less than200 mTorr.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is as follows:
 1. A method of depositing a dense aluminum oxide upon a substrate comprising the steps of:a) providing a vacuum processing chamber, the processing chamber having at least one gas injector nozzle for injection of gas into the chamber, a heated susceptor upon which is placed said substrate, and temperature controllable walls; b) providing a chemical vapor delivery system having a premix chamber connected to the processing chamber, the chemical vapor delivery system controllably generating and delivering a chemical vapor from a source of aluminum tri-isopropoxide (ATI); c) heating the ATI source to a temperature sufficient to provide a flow of the chemical vapor through the chemical vapor delivery system by direct evaporation of ATI; d) pumping out said chemical vapor from the ATI source, bypassing the processing chamber, until a predetermined purity of the chemical vapor is detected; e) controlling the flow of the chemical vapor in the chemical vapor delivery system to the processing chamber at a predetermined flow rate when said predetermined purity is detected; f) maintaining a process pressure within the processing chamber and the chemical vapor delivery system at a prescribed pressure; g) mixing a preheated inert gas with the flow of chemical vapor in the premix chamber to produce a gas mixture, the flow of inert gas being mass flow controlled; and h) injecting the gas mixture into the processing chamber so that the chemical vapor in the gas mixture reacts on the substrate to form an aluminum oxide film thereon.
 2. The method of claim 1, wherein the temperature in step c) is between 115° and 125° C.
 3. The method of claim 2, further comprising the step of maintaining a temperature of the processing chamber to less than 5° C. above the temperature of step c).
 4. The method of claim 3, wherein the temperature in step c) is approximately 120° C. and the temperature of the processing chamber is less than 125° C.
 5. The method of claim 1, wherein the flow of said chemical vapor in step e) is at least 1 sccm.
 6. The method of claim 1, wherein the prescribed pressure in step f) is less than 200 mTorr.
 7. The method of claim 1, wherein a nominal rate of inert gas flow is 200 sccm.
 8. The method of claim 7, wherein the inert gas is Argon.
 9. A method to use aluminum tri-isopropoxide (ATI) for chemical vapor deposition (CVD) of aluminum oxide, said method comprising the steps of:a) storing ATI during periods of no production demand at a temperature no less than 90° C. but not greater than 95° C.; b) storing ATI during periods of no production demand under vacuum; c) raising the ATI temperature to a temperature no greater than 125° C. in preparation for a period of production demand; d) maintaining ATI during periods of production demand at a temperature not greater than 125° C.; e) transporting ATI to a CVD reactor via direct evaporation; f) agitating the ATI during periods of production demand; g) directing the ATI to a reactor bypass in lieu of to the CVD reactor during a start up period and maintaining flow to the bypass until the concentration of ATI has reached a predetermined purity; h) directing the ATI to the CVD reactor after the concentration of ATI has reached a predetermined purity for deposition of aluminum oxide; and i) reducing the ATI temperature to about 90° C. after termination of the production demand period.
 10. The method of claim 9, further comprising the step of maintaining the temperature of ATI during delivery prior to the period of production demand.
 11. The method of claim 10, further comprising the step of maintaining the temperature of ATI during steps e-h.
 12. The method of claim 11, wherein the pressure of the CVD reactor is less than an ATI vapor pressure. 